1. Field of the Invention
The present invention relates generally to the continuous handling of material for processing. More particularly, the present invention relates to a belt feed machine for trimming and forming leads on semiconductor electrical components.
2. Description of the Invention Background
Solid state electrical devices are typically connected to other devices, as well as common substrates, such as printed circuit boards, through the use of electrical connectors, or leads, that are attached to input and output contacts on the device. The quality of the electrical connections between the devices depends upon the proper formation and positioning of the leads and the proper placement of the device.
The individual electrical devices are typically mass produced on common semiconductor substrate, or wafer, which is subsequently cut up to separate the individual dies. Electrical leads are attached to the dies as part of a preformed lead frame in which the leads are flat members extending from a common paddle that is attached to the face of the die. The leads are subsequently trimmed from the lead frame and formed to the desired shape after attachment to the die. Lead frames are often produced as a series of individual frames, each containing electrical leads for attachment to a die. The formation of multiple devices in a single lead frame or strip provides for easier handling of the lead frame during processing. In addition, the lead frames typically contain indexing holes for use in handling and alignment of the lead frame during subsequent processing. After the leads are attached, the devices are typically encapsulated in a molding compound to protect the device from moisture and other deleterious environmental conditions. The lead frames also contain dambars that are attached perpendicularly to the leads to provide structural support to the leads during processing and to prevent molding compound that extrudes from the mold during the encapsulation, known as flashing, and accumulates between the leads from flowing onto the portion of the leads to be attached to another component or onto adjacent devices.
After the plastic encapsulation of the device, the flashing and the dambars must be removed from between the leads. In addition, the electrical leads must be disconnected from the lead frames, trimmed and formed to a desired shape. Finally, the individual devices must be separated from the lead frame to yield the finished product. Each of these processes is generally performed through the use of die and punch tooling.
In the prior art, specially dedicated machines were used to perform each of the die and punch operations. The strips of lead frames would be processed in one machine for a given step and then transported to another machine to further processing. However, the transporting of the strips between machines and the required overhead with loading and feeding strips to the machines greatly increased the processing time and lowered the yield of the devices due to higher incidence of damage. Many of the problems with the use of the individual machines were overcome with development of integrated machines that can be used to perform a series of tooling operations on the framed device in one machine. In those machines, the die and punch tooling operations are linearly arranged in tooling stages and the frames are moved serially through each tooling operation.
The integrated machines use a "walking beam" method to advance the frames through the various stages. In a walking beam method, the lead frame or strip is fed into a track at the inlet of the machine with the lead frame and the faces of the devices in a horizontal orientation. The track supports the edges of the frame while leaving both faces of the device exposed and provides a guide for the strip through the machine as the strip is advanced by fingers extending from the walking beam. When the indexing holes on the lead frame reach the initial position of the first finger of the walking beam, a first set of pins extending from the first finger engage the indexing holes in the lead frame. Actuation of the beam causes the finger to move the lead frame to the first tooling stage. In the tooling stages, the punch tooling is reciprocated to contact and push the lead frame from above so as to disengage the lead frame from the pins on the walking beam finger and to push the lead frame onto the alignment pins attached to the stationary die. Once the lead frame is seated with the alignment pins in the indexing holes, the punch tooling stroke is continued to perform the tooling operation on the device. After the punch tooling disengages the lead frame from the walking beam finger pins, the finger is reciprocated back to its initial position where the pins on the finger engage the next pair of indexing holes in the lead frame, while during the punch operation is occurring. After the punch operation is completed, the punch tooling is reciprocated away from the stationary die and the track and lead frame lift off of the alignment pins on the stationary die. The walking beam finger is then actuated to advance the next frame into the tooling stage, which advances the preceding frame into the next tooling stage. In the final step, the devices are removed, or singulated, from the frames and the frames are discarded. While the use of the walking beam has provided a significant improvement over the prior art, the overall throughput of the machines is limited by the number of times that the strip must be engaged and disengaged by the walking beam pins, which is one of the most time consuming operation during processing. Also, the necessary reciprocal motion of the actuator results in a significant amount of unnecessary machine operations that can affect the long term reliability of the machine. Additionally in the walking beam method, the punch tooling is reciprocated not only to bring the punch into contact with the device, but to align and drive the device into the die tooling. This procedure significantly increases the stroke length of the punch, thereby increasing the possibility of damaging the devices, in addition to potentially causing tooling alignment difficulties due to bending of the frames and/or track.
Some of the problems associated with the unnecessary machine motion and potential overstroke of the punching tooling are resolved with the development of the pinch roller advance machines. The pinch roller machine advances the strip in a vertically oriented position through the use of a series of pinch rollers that contact the edges of the lead frame. The only advancement operation performed by the pinch roller machine operation is the rotation of the pinch rollers to advance the strip, thereby eliminating the unnecessary reciprocal operations associated with the walking beam method. Additionally, the pinch roller machine provides for reciprocal movement of both the punch and die tooling so as to reduce or eliminate many of the problems associated with the movement of only the punch tooling in the walking beam method. However, a limitation the pinch roller method is that the rollers must still be disengaged to some extent in each tooling stage to allow the alignment of the lead frame on the alignment pins of the die tooling prior to performing the tooling operation. Unlike the walking beam method, the disengagement of the strip by the rollers and the alignment of the frame on the die are not inherently interrelated operations, and therefore, must be synchronized to operate correctly, such as through the use of computer controller. The same is true after the completion of the tooling operation and the reengagement of the strip by the pinch rollers. As is the case with the walking beam method, these operations are a critical path operation and tend to limit the throughput of the machines. In addition, the performance of the pinch rollers must be closely monitored to ensure that the rollers do not apply excessive compressive forces on the lead frame during movement of the strip that may tend to damage frame, but that sufficient force is applied to prevent the strip from slipping during rotation of the roller that will cause a misalignment condition.
The present invention is directed to continuous belt feed design which overcomes, among others, the above-discussed problems so as to allow machines that commonly use walking beam transfer arrangements to provide for increased throughput capacities by eliminating the unproductive and time consuming machine operations that are required to reciprocate the walking beam apparatus back into position prior to handling subsequent devices.